Multi-color Cholesteric Liquid Crystal Display (LCD) devices are generally constructed with at least two glass-encapsulated parallel layers of disparate Liquid crystal materials wherein each layer has a respectively parallel addressing scheme of substantially transparent conductor to predetermined regions in each layer. Cholesteric liquid crystal display devices can be either mono-color or multicolor.
Mono-color devices typically have one liquid crystal layer and multicolor typically have two or more liquid crystal layers. Each of these predetermined layers has an upper glass conductor location and an opposite lower glass conductor location such that a specific voltage even between the two will “drive” the liquid crystal material there-between into a desired optical state. Three such optical states are commonly encountered and are planar, Homeotropic, and focal-conic.
Most current liquid crystal displays use a nematic-based technology in which two states are used. The twisted nematic device is the most common. One state is the field-applied state and the second is a non-field applied state. In most cases, the field on state consists of molecules that are aligned with their long molecular axis parallel to the electric field direction (and induced Homeotropic State). The field off state is an aligned homogeneous state (long molecular axis parallel to the glass substrates). Both states are optically transparent. To achieve gray level intermediate states intermediate voltage levels are used. The position of the molecules is observed by using polarized light and the molecules of the liquid crystal phase act as variable “retarders” to the polarized light. It is generally desired in the LCD industry to achieve high graphic resolution (ultra small pixels), video refresh rates, variable “gray” levels for each color layer, and to use low voltage.
Bi-stable display modes such as the cholesteric liquid crystal display traditionally operate between two stable states (planar and focal conic); an applied electric field is used to transfer between these states usually via a meta-stable stable (Homeotropic State). The planar state consists of an aligned helix of molecules the molecules lie substantially parallel to the glass substrates and it reflects light of a specific wavelength that is proportional to the pitch length of the cholesteric liquid crystal. Any light scattering that does occur is not intended and the phase is substantially transparent. The focal conic state consists of many randomly arranged cholesteric helices that are too small to reflect light but can cause scattering of incident light. This state is therefore not transparent but slightly light scattering. This reduces the contrast and limits the color gamut in multicolor devices; thus, the colors are less vivid. The contrast between these two states is emphasized by the use of a black absorber placed behind the layer(s) of liquid crystal that absorbs transmitted light. The observer then either sees only the reflected ray from the planar texture or the black absorber that is apparent when the cholesteric liquid crystal is in the focal conic texture. Intermediate gray levels arise from areas that are brought into a state that has both states present. If several different color layers are used and each is in the planar texture wavelengths from the entire visible spectrum can be reflected and the device appears white. By selecting the layers that will reflect many colors can be reflected including gray levels of these colors.
It is an amazing historical anomaly that in such displays the two stable states have been almost exclusively used.